In an aviation communication and/or navigation radio receiver, it can be advantageous to demodulate and decode information from more than one channel, all of which are present within a wideband radio frequency (RF) signal received from a single antenna, at the same time. Various architectures and methods for implementing such a system have been previously described and/or developed. In one approach, a continuous radio frequency (RF) band of aviation communication and/or navigation signals are simultaneously converted to the digital domain with a high-speed analog-to-digital converter. A digital processing system can then simultaneously select, process, decode, and output useful information to an aircrew from two or more of the channels present in the RF communications and/or navigation band.
One of the challenges with this architecture is the needed dynamic range for a single wideband receiver to operate in the presence of all the possible signal strengths present in a continuous band. Commercial cellular wireless systems avoid this problem by using one of two techniques: 1) they use separate bands for transmit and receive operation (frequency division duplex operation) and use power control so that the transmitters adjust their power based on how close they are to the receiving base station, thus ensuring that all signals received by the base station are near equal signal strength; or 2) they use time division duplex operation so that all mobile users transmit on time slots assigned for transmit with power control based on how close they are to the base station and receive on time slots assigned for base station transmissions. Thus, commercial wireless cellular base stations may use a single wideband receiver architecture to process digitally the signals received from multiple mobile cell phones and not have to deal with the large differences in the strength of the signals present in the entire receive band or receive time slots.
Perhaps the worst case known to occur is in the aeronautical Very High Frequency (VHF) Communications band of 118-137 MHz, where the same channels/frequencies are used for transmit and receive operation in a half-duplex mode of operation and where coordination between transmitters and receivers is not possible. The dynamic range of the signals present in the VHF communications (COM) band is excessive because an aircraft will have more than one VHF COM transceiver channel active at the same time. When one transceiver transmits (typically on its own antenna), an extremely strong VHF COM signal will be present in the VHF COM frequency band seen by a second VHF COM transceiver (typically on a second antenna). The second VHF COM transceiver is required to receive what may be a very weak signal from a distant ground station, even though the first VHF COM transceiver is driving an extremely strong interfering signal in the band and the frequency separation between the strong and the weak signals may be only a couple of channels.
In a typical single-channel VHF COM transceiver, this condition is addressed with a combination of tunable (or selectable) band-pass filters prior to a mixing stage, followed by a narrow-band single-channel fixed frequency band-pass filter after the mixing stage. The mixing stage is configured to pass the desired channel through the narrow-band fixed frequency band-pass filter rejecting all other signals in the band, prior to processing the desired signal. This architecture, though effective for strong signal rejection, can only receive one channel at a time from among all the channels in the VHF COM band.
One approach to mitigate the effects of a strong interfering signal is the use of tunable RF band-stop (wide notch) filters. These filters can be adjusted by the digital system to reduce the strength of the strong interferer without overly attenuating a weak desired signal. While this approach is somewhat effective, it still does not allow for the full desired performance. Specifically, the selectivity of a tunable RF notch filter implemented with lumped circuit technology is too wide and may attenuate a desired signal too much when the strong interferer is close in frequency. In addition, the transition band of tunable filters is broader than is acceptable to meet the performance expectations of some aircraft operators.